CHAP, iv.] THE VASCULAR MECHANISM. 287 



when we apply a weak stimulus we get a small contraction ; within 

 certain limits (see 79) the contraction is proportional to the 

 stimulus. This is not the case with the quiescent ventricle or 

 heart. j^When we apply a strong induction-shock we get a beat of 

 a certain strength ; if we now apply a weak shock we get either no 

 beat at all or quite as strong a beat as with the stronger stimulus. 

 That is to say the magnitude of the beat depends on the condition 

 of the ventricle (or heart) and not on the magnitude of the stimulus. 

 If the stimulus can stir the ventricle up to beat at all, the beat is 

 the best which the ventricle at the time can accomplish ; the 

 stimulus either produces its maximum effect or none at all. It 

 would seem as if the stimulus does not produce a contraction in 

 the same way that it does when it is brought to bear on a skeletal 

 muscle, but rather stirs up the heart in such a way as to enable it 

 to execute a spontaneous beat which, without the extra stimulus, it 

 could not bring about. And this is further illustrated by the fact 

 that when a ventricle is beating rhythmically either spontaneously, 

 or as the result of rhythmic stimulation, the kind of effect produced 

 by a new stimulus thrown in will depend upon the exact phase of the 

 cycle of the beat at which it is thrown in. If it is thrown in just 

 as a relaxation is taking place, a beat follows prematurely, before 

 the next beat would naturally follow, this premature beat being 

 obviously produced by the stimulus. But if it be thrown in just 

 as a contraction is beginning, no premature beat follows : the ven- 

 tricle does not seem to feel the stimulus at all. There is a period 

 during which the ventricle is insensible to stimuli, and that how- 

 ever strong ; this period is called the " refractory " period. (There 

 is it may be mentioned a similar refractory period in skeletal 

 muscle, but it is of exceedingly short duration.) From this it 

 results that, when a succession of stimuli repeated at a certain rate 

 are sent into the ventricle, the number of beats does not correspond 

 to the number of stimuli, some of the stimuli falling in refractory 

 periods are ineffective and produce no beat. Hence also it is 

 difficult if not impossible to produce a real tetanus of the ventricle, 

 to fuse a number of beats into one. And there are other facts 

 tending to shew that the contraction of a cardiac muscular fibre, 

 even when induced by artificial stimulation, is of a peculiar nature, 

 and that the analogy with the contraction of a skeletal muscular 

 fibre, induced by motor impulses reaching it along its nerve, does 

 not hold good. 



These and other considerations, taken together with the facts 

 already mentioned that portions of cardiac muscular tissue in 

 which ganglionic cells are certainly not present can, in various 

 animals be induced, either easily or with difficulty, to execute 

 rhythmic beats, which have all the appearance of being spontaneous 

 in nature, lead us to conclude that the beat of the heart is not the 

 result of rhythmic impulses proceeding from the cells of the ganglia 

 to passive muscular fibres, but is mainly the result of changes taking 



